The disclosed technology relates generally to methods and tools for micro-transfer-printing. Conventional methods such as pick-and-place for applying integrated circuits to a destination substrate are limited to relatively large devices. For example, having a dimension of a millimeter or more and it is often difficult to pick up and place ultra-thin, fragile, or small devices using such conventional technologies. More recently, micro transfer printing methods have been developed that permit the selection and application of these ultra-thin, fragile, or small devices without causing damage to the devices themselves.
Micro-transfer-printing enables deterministically removing arrays of micro-scale, high-performance devices from a native source wafer, typically a semiconductor wafer on which the devices are constructed, and assembling and integrating the devices onto non-native destination substrates. In its simplest embodiment, micro-transfer-printing is analogous to using a rubber stamp to transfer liquid-based inks from an ink-pad onto paper. However, in micro-transfer-printing, the “inks” are composed of high-performance solid-state semiconductor devices and the “paper” can be substrates, including glass, plastics, or other semiconductors. The micro-transfer-printing process leverages engineered elastomer stamps coupled with high-precision motion-controlled print-heads to selectively pick-up and print large arrays of micro-scale devices from a source native wafer onto non-native destination substrates.
Adhesion between the elastomer transfer device and the printable element can be selectively tuned by varying the speed of the print-head. This rate-dependent adhesion is a consequence of the viscoelastic nature of the elastomer used to construct the transfer device. When the transfer device is moved quickly away from a bonded interface, the adhesion is large enough to “pick” the printable elements away from their native substrates, and conversely, when the transfer device is moved slowly away from a bonded interface the adhesion is low enough to “let go” or “print” the element onto a foreign surface. This process may be performed in massively parallel operations in which the stamps can transfer, for example, hundreds to thousands of discrete structures in a single pick-up and print operation.
Micro transfer printing enables parallel assembly of high-performance semiconductor devices onto virtually any substrate material, including glass, plastics, metals, or semiconductors. The substrates may be flexible, thereby permitting the production of flexible electronic devices. Flexible substrates may be integrated in a large number of configurations, including configurations not possible with brittle silicon-based electronic devices. Additionally, plastic substrates, for example, are mechanically rugged and may be used to provide electronic devices that are less susceptible to damage or electronic performance degradation caused by mechanical stress. Thus, these materials may be used to fabricate electronic devices by continuous, high-speed, printing techniques capable of generating electronic devices over large substrate areas at low cost (e.g., roll-to-roll manufacturing).
Moreover, micro transfer printing techniques can print semiconductor devices at temperatures compatible with assembly on plastic polymer substrates. In addition, semiconductor materials may be printed onto large areas of substrates thereby enabling continuous, high-speed printing of complex integrated electrical circuits over large substrate areas. Fully flexible electronic devices with good electronic performance in flexed or deformed device orientations can be provided to enable a wide range of flexible electronic devices.
Micro-structured stamps may be used to pick up micro devices from a source substrate, to transport the micro devices to the destination, and to print the micro devices onto a destination substrate. The transfer device (e.g., micro-structured stamp) can be created using various materials. Posts on the transfer device can be generated such that they pick up material from a pick-able object and then print the material to the target substrate. The posts can be generated in an array fashion and can have a range of heights depending on the size of the printable material. For effective, high-yield printing, when picking up the material it is important that stamp posts are in close contact with the material (e.g., micro integrated circuits) being transferred or printed. However, many integrated circuits do not have a planar surface whose area is readily contacted by a stamp post.
There is a need, therefore, for stamps having an improved ability to pick up and transfer material with a non-planar surface.